This invention relates to arc spinner type circuit interrupters and more specifically relates to a novel improved arc spinner interrupter in which the coil of the arc spinner which induces a circulating current in the arc runner of the interrupter is enclosed by a relatively high permeability magnetic material which is saturable at relatively high coil current. The interrupter then exhibits improved interrupting characteristics at short-circuit currents which are relatively low compared to the rated short-circuit capabilty of the device without substantially increasing mechanical stresses within the assembly due to repulsion forces between the coil and the short-circuited arc runner at rated short-circuit current.
Arc spinner type interrupters are well known and typical prior art devices are disclosed in U.S. Pat. No. 4,052,577 in the name of Gerald A. Votta and U.S. Pat. No. 4,052,576 in the name of Robert Kirkland Smith.
In the arc spinner type interrupter, an arc is drawn between a circular arc runner and a relatively movable contact which moves into and out of engagement with the arc runner. The arc runner and movable contact are contained in a dielectric gas-filled housing. The gas may be sulfur hexafluoride or any other desired dielectric gas. The disk-shaped arc runner is closely magnetically coupled to a series-connected coaxial coil which carries the arc current and which also induces a circulating current in the arc runner which is formed in the manner of a short-circuited turn. The magnetic field produced by the circulating current in the arc runner and by the coil interact with the arc current in the arcing space between the contacts to create a Lorentz force which tends to rotate or spin the arc around the arc runner and relative to the dielectric gas which fills the arc space. The relative motion between the arc and the gas then causes the cooling and deionization of the arc to allow extinction of the arc when the arc current passes through zero.
All prior designs of the coil of an arc spinner interrupter use basically non-magnetic materials. Consequently, the force on the arc which tends to rotate the arc in the gas filling the arc space near arc current zero is a function of the RMS value of the current being interrupted.
Arc spinner interrupters using a coil composed of non-magnetic material have the ability to withstand a rapidly rising transient recovery voltage (TRV). This ability increases as the short-circuit current magnitude increases. However, as will be later described more fully, the TRV recovery rate required by ANSI standards is higher at low current that at high current. Consequently, a particular coil design which will meet the TRV requirements at high currents normally cannot interrupt the required TRV at low currents. This is because of the linear relationship between magnetic flux density and short-circuit current possessed by a coil composed of non-magnetic material.
The interrupting performance of the arc spinner interrupter can be improved by adding additional turns to the coil so that at low current a high enough magnetic flux will appear in the arc current space to produce the desired current interruption. Thus magnetic flux density is directly related to the number of turns used in the coil, and increasing the flux density will also increase the interrupting ability of the device. However, increasing the number of turns of the coil will increase the repulsion forces between the coil windings and the short circuited circular arc runner as the square of the current and the square of the number of coil turns. Thus increasing the number of turns of the coil to meet low current interrupting goals results in greatly increased repulsion forces at high currents. These forces can be large enough at high currents to deform the metal parts or break them and destroy the coil.
Consequently, the designer is faced by the dilemma that additional coil turns are required to meet low current interrupting goals, but fewer coil turns are required to keep repulsion forces between the coil and the arc runner within reasonable structural strength limits.